Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for MPOL_PREFERRED_MANY by adding a NO_SLOWPATH gfp bit

[Dave Hansen]

On 3/3/21 8:31 AM, Ben Widawsky wrote:
>> I haven't got to the whole series yet. The real question is whether the
>> first attempt to enforce the preferred mask is a general win. I would
>> argue that it resembles the existing single node preferred memory policy
>> because that one doesn't push heavily on the preferred node either. So
>> dropping just the direct reclaim mode makes some sense to me.
>>
>> IIRC this is something I was recommending in an early proposal of the
>> feature.
> My assumption [FWIW] is that the usecases we've outlined for multi-preferred
> would want more heavy pushing on the preference mask. However, maybe the uapi
> could dictate how hard to try/not try.

There are two things that I think are important:

1. MPOL_PREFERRED_MANY fallback away from the preferred nodes should be
   *temporary*, even in the face of the preferred set being full.  That
   means that _some_ reclaim needs to be done.  Kicking off kswapd is
   fine for this.
2. MPOL_PREFERRED_MANY behavior should resemble MPOL_PREFERRED as
   closely as possible.  We're just going to confuse users if they set a
   single node in a MPOL_PREFERRED_MANY mask and get different behavior
   from MPOL_PREFERRED.

While it would be nice, short-term, to steer MPOL_PREFERRED_MANY
behavior toward how we expect it to get used first, I think it's a
mistake if we do it at the cost of long-term divergence from MPOL_PREFERRED.